1. Field of the Invention
The present invention relates to a source sheet for stencil painting, a method of manufacturing a plate for stencil printing from the source sheet, and a stencil printing method in which the plate is used.
2. Description of the Related Art
As a source sheet for stencil painting (stencil source sheet), a heat-sensitive source sheet for the stencil printing perforated by infrared irradiation or a thermal head has heretofore been known. The source sheet obtained by attaching a thermoplastic film and porous tissue paper to each other by an adhesive has been for general use.
Moreover, as a stencil printing apparatus in which the heat-sensitive source sheet is used, mainly a rotary stencil printing apparatus and simple press type stencil printing apparatus are known.
In these printing apparatuses, ink is pushed out from a tissue paper side of the source sheet through pores made in the film corresponding to image area, and transferred onto a printing sheet so that the printing is performed.
In a conventional stencil printing system, much time is required for ink to permeate through the printing sheet, and therefore there has been a demand for improvement in an ink drying property.
That is, the ink does not easily permeate through the printing sheet. This causes a problem that fingers are stained upon touching a printed matter immediately after the printing. As another problem, when the printing of second and subsequent-color in a multicolor printing or the printing of a back surface in a double-surface printing is continuously performed, the ink on an insufficiently dried printing sheet is transferred to a rubber roll of a printer, the ink is again transferred to the next printing sheet, and the printed sheet is made dirty. This further causes a problem that a long time (e.g., about 10 to 20 minutes) is taken for shifting to the next step in order to sufficiently dry the sheet.
Here, in order to enhance the drying property of the ink, it is effective to use a low-viscosity ink and enhance permeability of the ink into the printing sheet.
However, when the low-viscosity ink is used, but when an ink transfer amount is excessive, the drying property is deteriorated. Therefore, when the low-viscosity ink is used in the conventional stencil printing system, it is necessary to set a perforation diameter to at least 20 μm or less in order to control the ink transfer amount.
However, when the perforation diameter is reduced as described above, a perforated dot density needs to be raised in order to prevent the image area from thin spots. For this, it is necessary to raise a heating element density (resolution) of the thermal head. This requires not only cost increase of the thermal head, but also remarkable level enhancement of peripheral techniques such as the securing of durability of the thermal head, enhancement of yield, and increase of film sensitivity of the heat-sensitive source sheet.
To solve the above-described problems, the present inventors have proposed a stencil source sheet and printing method in which a micro porous plastic sheet (hereinafter referred to as the micro porous sheet) with micro continuous pores formed beforehand therein by a submicron unit is used to block off pores corresponding to non-image area and thereby a portion prohibiting passage of ink is formed (Japanese Patent Application No. 2000-188504).
However, a manufacturing process of the above-described micro porous sheet is complicated, and much time is required for forming the micro pores in the sheet. Therefore, there are problems that a film forming rate is very slow, productivity is deteriorated, and the process is economically insufficient.
Furthermore, since the manufacturing process of the micro porous sheet includes an extension process in forming the films, the sheet has a property of easily thermally contracting by heating. Therefore, the micro porous sheet is thermally deformed more than necessary by the heating by the thermal head in manufacturing a plate. There is a problem that a dimension reproducibility in manufacturing the plate is deteriorated.
As described above, in the stencil printing, it has been difficult to satisfy both image properties such as the preventing of the image area from thin spots, and quick-drying properties.